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Lecture Comments (3)

2 answers

Last reply by: Nagasrinivas Tripuraneni
Tue May 12, 2015 8:06 AM

Post by Nagasrinivas Tripuraneni on May 11, 2015

Professor, can you explain about flash point ?

Introduction to Melting Points

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Definition of a Melting Point (mp) 0:04
    • Definition of a Melting Point (mp)
    • Solid Samples Melt Gradually
    • Recording Range of Melting Temperature
  • Melting Point Theory 3:14
    • Melting Point Theory
  • Effects of Impurities on a Melting Point 3:57
    • Effects of Impurities on a Melting Point
    • Special Exception: Eutectic Mixtures
    • Freezing Point Depression by Solutes
  • Melting Point Uses 6:19
    • Solid Compound
    • Determine Purity of a Sample
    • Identify an Unknown Solid
  • Recording a Melting Point 9:03
    • Pack 1-3 mm of Dry Powder in MP Tube
    • Slowly Heat Sample
    • Record Temperature at First Sign of Melting
    • Record Temperature When Last Crystal Disappears
    • Discard MP Tube in Glass Waste
    • Determine Approximate MP
  • Tips, Tricks and Warnings 12:28
    • Use Small, Tightly Packed Sample
    • Be Sure MP Apparatus is Cool
    • Never Reuse a MP Tube
    • Sample May Decompose
    • If Pure Melting Point (MP) Doesn't Match Literature

Transcription: Introduction to Melting Points

Hi and welcome to

Today, we are going to be talking about melting points.0002

When you think about what happens when you melt a solid, we start out as a crystal structure of some kind,0006

where we have very close contact between all of our particles and very little motion.0012

As we heat the molecule, as we heat the simple, we increase the kinetic energy.0019

We start to get some liquor vibrations, some movements, some kinetic energy.0024

Eventually, when we reach the temperature we describe as the melting point, 0028

we overcome the energy that is holding the crystals together.0032

They release and become in the liquid form.0036

Now the molecules are still intact but they are now freely flowing and we have a lot more kinetic energy.0038

Notice this molecule being represented as the hexagon, it is still a hexagon.0046

We are not breaking any bonds within the structure.0050

We are simply disrupting the intermolecular forces that are holding one molecule together to another molecule.0053

If we have a pure solid, we would have just all of the same molecules repeating in our crystal structure.0061

When we hit it, we now have the pure liquid.0068

If we were to take a pure liquid and we were to cool it, and lower the temperature, lower the kinetic energy,0070

and finally we now do not have enough energy to overcome the crystal lattice strength.0076

The particles come together, drawn together by intermolecular forces.0083

We would end up in solid form again.0086

That is the overall process of melting, we need to keep that in mind.0088

The definition of our melting point is the temperature at which a solid changes to a liquid, that is one atmosphere.0093

It is the same temperature as the freezing point.0100

This temperature to do this transition is the same in both directions.0103

It is called the freezing point for the liquid going to the solid.0107

Solid samples melt gradually.0111

They kind of happen at the surface and wherever part of the crystal structure is reaching the proper temperature,0113

that melting is going to occur and transformation into the liquid.0120

When we take a melting point of a solid sample, we are going to be recording a range of melting temperatures.0125

It is not really a point that we record in our notebooks.0133

A range is going to be, for example, we might say the melting point of our sample was 72.6 - 74° C.0139

What those two numbers represent is the first number is the temperature at the very first sign of melting.0146

When we see the bulk of our crystal starts to look wet, that means it is starting to melt.0151

And then, we are going to continue watching our sample.0157

When finally the last crystal melts and all we have is a liquid remaining in our melting point tube,0159

that is the final temperature we are going to record.0165

That is described as a melting point range.0167

If we have a very pure compound, typically, its range is a sharp one.0171

We only have 1 - 2° difference between the first number and the second number.0175

In the literature it is going to be reported as just a single number.0181

You see this higher number that is reported, that should be the temperature at which your sample has completely melted,0185

if you would record the melting point.0192

How is it that the range is a measure of the samples purity?0197

We know that in a solid, our molecules are going to be organizing a network of crystals.0201

As a sample is heated, our kinetic energy increases.0207

We just talked about this a little bit.0209

Energy, ultimately, is required to break the crystal lattice.0211

As our motion increases, we overcome that energy and molecules break free.0216

During the melting, we have our solid and our liquid phases are in equilibrium, as that process is happening.0220

And then finally, once we have completely melted,0228

we have a liquid with increased motion and less contact between the molecules.0230

It is kind of the theory behind the picture that we just had on the last slide.0234

What happens if we have a small amount of organic impurities mixed into our crystals?0238

They are going to disrupt the crystal structure.0244

Instead of having that beautiful structure where we have them homogenously connected.0247

If we now have something in here that does not fit the crystal structure, 0252

that is going to be a disruption of the overall structure.0256

That is going to be our impurity.0263

If we want to melt this solid, it is not going to take as much heat to break that crystal structure 0266

because there is already some disruptions to the crystal lattice.0271

Typically, the melting point temperature, if we have a small amount of organic impurities,0275

typically, the melting point temperature is going to be lower than what we expect in the literature.0280

The range is going to be wider.0287

Instead of having a sharp melting point range, meaning just 1 or 2° difference between the first number and the second number, 0289

we are going to have a wider range, maybe 3° or 5° or 10° or 20°.0295

It really depends on what your impurity is and how much you have of it, and so on.0300

This is not always the case.0304

Again, what I'm talking about are generalizations and this is typically true.0306

But there are special mixtures, they are described as eutectic mixtures.0310

It is like an azeotrope, which is a mixture of liquids that happen to boil at a steady temperature, a single temperature.0314

We can have that same special relationship with certain solids, when they are mixed.0321

They will behave as impure compounds.0325

They will have a sharp melting point even though it is a mixture of compounds.0328

Again, there are exceptions to this, but in general,0331

we find that the impurities causing it to be lower than the literature value and with a wider range.0334

This is the same concept that we have for freezing point depression.0340

You may recall that by adding a solute to a liquid, it causes the freezing point to lower.0343

This is how it works when we put rock salt on our sidewalks, if it is cold out and they are frozen.0349

Putting rock salts on there causes the ice to melt because now you have brine, 0356

you have a salt solution that has a lower freezing temperature.0361

It is going to remain a liquid at a lower temperature.0365

It is the same thing that we are having a lower temperature of melting.0367

It is the same thing as having a lower temperature of freezing, by adding a solute, 0371

adding an impurity into your pure compound.0375

How might we use a melting point in the lab?0381

There are several things you can do.0383

One of them is -- this is for sure one thing we are going to do.0384

If we ever come across a solid compound and unknown that we are trying to identify,0388

or a product, if the reaction that we have run...0392

Definitely, one of the things we want to do to characterize that is to measure its melting point.0394

That tells us something about the solid that we have.0399

Another thing is that we can get some indication of the purity of our sample, when we record the melting point.0402

If it is a sharp melting point 1 - 2°, then it is likely to be a pure compound.0408

Again, there are exceptions but those are quite rare.0414

If we have a wide range, it is a likely to be impure.0417

That is an indication that you have your solid here but there is probably something else mixed in there, 0420

because we do not have our sharp melting point.0425

We can also use it to identify an unknown solid.0428

These top two, a lot of this is just kind of verifying that I did a reaction, I think this is the compound that I made.0432

I can use melting point to verify that yes, that is the compound I made.0440

And yes, I have properly purified it.0444

But if we have an unknown solid, we can actually help identify it by its melting point.0446

That kind of narrows down your choices of what it can possibly be, if we know what the melting point is.0453

But of course, there are hundreds of compounds that have the same melting point, as your unknown does.0458

If we happen to have a pure sample of what we think our unknown might be, 0463

we can use that to for sure identify our unknown, by doing what is called a mixed melting point.0469

If we have a standard, let us say I think my sample is benzoic acid 0476

and then I have some pure benzoic acid from the stockroom.0480

What I can do is I can take a mixture, I can combine my unknown and the benzoic acid sample, 0483

grind it up on a watch glass to make a homogeneous mixture powder.0490

Take a melting point of that.0494

And then, if there is no change in the melting point then that confirms that my unknown must be the standard.0495

Because when I have made a mixture, it still had a sharp melting point which means it was in fact not a mixture.0506

It was the same compound mixing with the same compound.0512

But here, if our melting point changes, then that confirms that our unknown compound is not the same as the standard.0514

Because now by mixing them one by one, you have introduced a large amount of impurity now.0524

Now the melting point is going to be wide range, much lower.0530

And then, all of the sudden we know that is a way of confirming0533

whether not a compound matches a standard that we might have.0538

How do we record a melting point?0544

There are a few steps we do.0546

The first thing we do is we pack… This is a melting point tube.0548

It is a very tiny, thin capillary tube, that is sealed off at one end.0551

We are going to make our sample a powder.0555

If it is not already a powder, you are going to grind it up to be a powder.0559

We are going to grind it up.0562

We are going to scoop a little tiny bit into the top.0564

We are going to tap it down to move it down to the bottom of the test tube.0566

And then, we are going to tap, continue tapping to try and get it as tightly packed as possible.0570

Our goal is this height should be from 1 to 3 ml.0576

If you do not know what that looks like, you should get a ruler to check it out and confirm.0582

We want this to be tightly packed from 1 – 3ml.0586

It will ensure that we have enough sample to observe and that will ensure even heating,0588

if it is tightly packed without any air pockets.0593

We are going to slowly heat the sample.0596

What does it mean to slowly heat?0598

Maybe 1 - 2° per minute.0600

This is not really an exciting thing to do in the lab because it requires some waiting around.0601

We want it to be slowly heated so we do not have a heat lag0607

between what the melting point apparatus says and what our crystals are actually heating at.0610

We want it to be slow enough so that as we are watching something happening, 0615

we have time to go and record the temperature and have it still be the right temperature.0619

What we are doing is we are watching the crystals.0624

The melting point apparatus typically has a magnifying glass and a light, so that our crystals are well illuminated.0627

We can watch them very carefully.0633

We are going to be watching that, also keeping an eye on the temperature.0635

What we are going to do is we are going to record that first number, when we see our first sign of melting.0638

Here is our crystals and there is a little part of it that is starting to look wet in the bulk of the crystal.0646

Sometimes our crystals shrink a little bit or shift a little bit, that does not mean it is melting, we ignore that.0652

But we want to have in the bulk of the sample, we want to look for a real wetness.0658

That is going to be our first number.0662

We are going to record that number.0664

As we are taking a melting point, we always have a pen in hand and 0665

we have our notebook right here so we can record it as it is happening.0667

We are going to continue watching and slowly but surely these are all going to start melting and getting wet.0671

Until finally, we have a total liquid in the melting point tube.0678

There are no crystals left anymore, we just have a liquid.0684

That temperature, we are going to record again and that is the second number that we report in our range.0686

Our melting point tube can now be discarded.0693

There is nothing left to do with it.0695

Make sure it goes in the glass waste because we do not want that to be hazard to anyone who handles the trash.0696

Sometimes because if you are at a very high melting point, it could take a long time to reach that, if you are heating it 1 - 2°.0703

Sometimes, you want to heat up the melting point apparatus quickly.0712

Let us say you are expecting it to melt around 180.0718

Maybe you can heat it up quickly to 150 and then slowly watch it from there on out.0720

Sometimes what you might want to do is do a quick melting point, a rough melting point,0726

by heating it much more quickly, maybe 5 - 10° per minute, to get an approximate melting point.0732

Now you know what range it is in, now you can watch that range more slowly.0738

That can you save you a little time, when you are doing your melting points.0744

What are some things that you should look out for, that will help you have a successful melting point recording?0749

Make sure it is a small sample and it is also tightly packed.0755

If you do not make sure it is a powder, you are not going to be able to get that done, that is important.0760

Make sure your melting point apparatus is cool, before you insert your sample.0766

A lot of times, someone just finished up with the apparatus and it is already at 150°.0770

It is off but it takes a while for that heating block to cool down.0775

Pick up your melting point tube, you look at it and it is already a liquid.0779

Throw that out in the glass waste and start a new one.0783

This time, wait patiently for it to cool a little bit.0788

Take a quick look at the thermometer before you insert your sample to make sure it is not already too hot.0791

Never reuse a melting point tube, once it turns into a liquid.0797

You can no longer see that powder that you need to analyze, to find that first melting point.0801

That is not appropriate anymore for any analysis, just discard that.0806

Sometimes you are looking for this wetness, you are looking for this melting.0812

And instead, maybe you see it getting dark or charring a little bit, 0815

that is a sign that your sample is decomposing, and that happens sometimes.0821

Rather than getting to a point where it melts, the structure itself breaks down.0826

That is where you actually having bonds in the structure, breaking and changing its chemical composition.0831

If that, in fact happens, that is okay.0837

You would just write the melting point, if maybe it started to melt or it just immediately started to char.0839

We use the letter D to indicate that decomposition has happened.0847

If you ever see that in the literature, that means this sample is something that is not stable to heating.0852

Instead of reaching a melting point, it decomposes instead.0858

What if you do your reaction and you know you isolate your compound, you purify it,0862

and you think you have what you have, but it does not match the literature value.0865

This is supposed to be your confirmation that your synthesis was successful.0870

There are a couple of things.0875

Make sure, did you really heat it very slowly?0876

Because if you do not heat it very slowly, your recorded melting point can be way off.0879

Maybe go back and repeat the melting point experiment, that is not a bad idea.0883

Maybe you have some solvent present that has not evaporated.0889

Make sure your sample is thoroughly dry, completely dry,0892

before you attempt to take the melting point because that is going to affect your melting point.0896

Maybe your apparatus needs calibrating.0901

They have corrected melting points where you calibrate the thermometer.0903

If your thermometer has not been calibrated, then that is something that can affect your melting point0909

and might be a reason that it is not matching the literature.0914

If you think it is close but I'm not sure you know that this is really the compound.0920

If you can obtain a pure sample of that compound then you can do the mixed melting point 0925

and that is going to be proof positive, one way or the other, 0931

whether yes it is the same compound because in the mixed melting point, your melting point does not change.0934

Or not for sure, this is not the right compound because the mixed melting point gives a big difference from the pure sample.0940

Of course, melting point is just one way to analyze a compound.0950

You can also confirm by TLC and maybe whether it is one or more spots,0953

you can do nanomer to see the purity of the compound and so on.0958

But melting point is a really quick and easy way to characterize a solid.0961

You should expect to do it any time we are dealing with a solid sample.0965

Good luck with your melting points.0969